Nuclear reactor refueling methods and apparatuses

ABSTRACT

Refueling of a nuclear reactor ( 40 ) includes removing a fuel assembly ( 10 ). The removal method includes lowering a lifting tool ( 80 ) of a crane ( 44 ) onto a top of the fuel assembly. The lowered lifting tool including a plurality of downwardly extending elements ( 82 ) that surround and vertically overlap a portion ( 74 ) of a control rod assembly ( 70 ) extending above the top of the fuel assembly. The downwardly extending elements are locked with corresponding mating features ( 26 ) at the top of the fuel assembly to connect the lifting tool with the fuel assembly. The connected fuel assembly is moved into a spent fuel pool ( 42 ) using the crane, and the lifting tool is disconnected from the top of the fuel assembly by unlocking the downwardly extending elements from the corresponding mating features at the top of the fuel assembly.

BACKGROUND

The following relates to the nuclear reactor arts, electrical powergeneration arts, nuclear reactor control arts, nuclear electrical powergeneration control arts, and related arts.

Nuclear reactors employ a reactor core comprising a critical mass offissile material, such as a material containing uranium oxide (UO₂) thatis enriched in the fissile ²³⁵U isotope. The fuel rod may take variousstructural configurations, for example including fissile material aspellets embedded in a ceramic matrix or so forth. To promote safety, itis conventional to assemble the core as rods containing the fissilematerial. A set of rods is preassembled to form a fuel assembly.Preferably, the mass of fissile material in the fuel assembly remainsbelow critical mass. The fuel assemblies are shipped to the reactorsite, and are installed in a grid in the reactor pressure vessel to formthe reactor core. To prevent a premature chain reaction, suitableneutron absorbing material is provided during installation, for exampleby inserting neutron-absorbing control rods into the fuel assembliesbefore they are brought together in the pressure vessel, and by omittingthe neutron moderator (e.g., water ambient) if employed.

With reference to FIGS. 1 and 2, an illustrative example of such anassembly is shown. FIG. 1 shows an illustrative fuel assembly 10including a set of fuel rods 12 secured together with a controlledspacing by mid-spacer grid elements 14 and by end-spacer grid elements16, 18. In the illustrative example, the fuel rods 12 form a 17×17array. The fuel assembly 10 is typically substantially elongated, and isshown in part in FIG. 1 with an indicated gap G. The fuel assembly 10also suitably includes other elements, such as control rod guide tubesor thimbles 20 through which neutron-absorbing control rods may pass.One or more of these or similar tubes or thimbles may also serve asinstrumentation conduits for in-core sensors. Upper and lower nozzleplates 22, 24 may be provided to facilitate coupling of control rods,instrumentation bundles, or so forth into or out of the fuel assembly10. The illustrative upper and lower nozzle plates 22, 24 includerespective upper and lower alignment pins 26, 28 at the corners of therespective nozzle plates 22, 24 for facilitating alignment of the fuelassemblies during installation in the reactor core.

FIG. 2 shows the assembled reactor core 30, including a closely packedgrid of fuel assemblies 10 disposed in a core former 32. In FIG. 2, acontrol rod assembly (CRA) is fully inserted into each fuel assembly 10.In the view of FIG. 2, only an upper support element 34 of the CRA isvisible extending above each corresponding fuel assembly 10. The uppersupport element of each CRA may in be a conventional spider or (as inFIG. 2) a larger element (see “Terminal Elements for Coupling ConnectingRods and Control Rods in Control Rod Assemblies For a Nuclear Reactor”,U.S. Ser. No. 12/862,124 filed Aug. 24, 2010, which is incorporatedherein by reference in its entirety, for some illustrative examples).The illustrative reactor core 30 includes sixty nine (69) fuelassemblies, although in general more or fewer fuel assemblies may beincluded.

The reactor core has a designed lifetime, typically in a range of a yearto a few years. The core lifetime is controlled by the reduction infissile material caused by operation of the nuclear chain reaction. Tocontinue operation, a refueling operation must be performed, in whichthe spent fuel assemblies are removed and replaced by new fuelassemblies. Typically, this entails shutting down the reactor, openingthe pressure vessel and removing any components in order to gainoverhead access to the fuel assemblies, and removing the fuel assemblieswith the assistance of a crane. To enable coupling with the fuelassembly, each fuel assembly is typically fitted with a box structurewith leaf springs mounted on top of the box, or a plate-and-poststructure with preloaded helical coil springs mounted between the posts.The fuel assembly is lifted by a grappling mechanism that engages thefixed top plate of the box structure or the movable top plate of theplate-and-post structure via hooks that swing laterally under the topplate in four orthogonal directions. In box designs, the hooks swingoutward to engage the top plate of the box, while in plate-and-postdesigns the hooks swing inward to engage the top plate.

These refueling approaches have substantial disadvantages. The swingingmotion of the grappling hooks calls for a large working space proximateto the top of each fuel assembly. However, this working space isconstrained by the presence of closely adjacent neighboring fuelassemblies in the array disposed in the core former. Moreover, if theCRA is left fully inserted into the fuel assembly during refueling(which is desirable to maintain suppression of the neutron population inthe fuel assembly during the refueling process), then either the spidermust be removed entirely (a process entailing individually detachingeach of the numerous control rods from the spider), or the spider mustbe of sufficiently low profile to enable the grappling hooks to operateabove the spider.

Disclosed herein are improvements that provide various benefits thatwill become apparent to the skilled artisan upon reading the following.

BRIEF SUMMARY

In one aspect of the disclosure, a method comprises performing refuelingof a nuclear reactor. The refueling includes removing a fuel assemblyfrom a reactor core of the nuclear reactor. The removal method includes:connecting a lifting tool of a crane with a top of the fuel assembly,the lifting tool comprising an assembly of downwardly extendingelements, the connecting including locking lower ends of the downwardlyextending elements with respective mating features located at a top andperiphery of the fuel assembly; moving the fuel assembly connected withthe lifting tool into a spent fuel pool using the crane; and releasingthe lifting tool from the top of the fuel assembly, the releasingincluding unlocking the lower ends of the downwardly extending elementsfrom the respective peripherally located mating features at the top andperiphery of the fuel assembly.

In another aspect of the disclosure, a method comprises performingrefueling of a nuclear reactor. The refueling includes removing a fuelassembly having a control rod assembly (CRA) inserted in the fuelassembly from a reactor core of the nuclear reactor. The removal methodincludes: lowering a lifting tool of a crane onto a top of the fuelassembly, the lowered lifting tool including a plurality of downwardlyextending elements that surround and vertically overlap a portion of theCRA extending above the top of the fuel assembly; locking the downwardlyextending elements of the lowered lifting tool with corresponding matingfeatures at the top of the fuel assembly in order to connect the liftingtool with the fuel assembly; moving the fuel assembly connected with thelifting tool into a spent fuel pool using the crane; and disconnectingthe lifting tool from the top of the fuel assembly in the spent fuelpool by unlocking the downwardly extending elements from thecorresponding mating features at the top of the fuel assembly.

In another aspect of the disclosure, an apparatus comprises a liftingtool including an upper end configured for attachment with a crane, anda plurality of downwardly extending elements surrounding an open centralregion disposed below the upper end, lower ends of the downwardlyextending elements being configured to mate with mating features at thetop of a fuel assembly of a nuclear reactor core.

In another aspect of the disclosure, an apparatus comprises: a nuclearfuel assembly including mating features at a top of the nuclear fuelassembly; and a lifting tool including an upper end configured forattachment with a crane and a plurality of downwardly extending elementssurrounding an open central region disposed below the upper end, lowerends of the downwardly extending elements being configured to mate withthe mating features at the top of the nuclear fuel assembly.

In another aspect of the disclosure, an apparatus comprises: a nuclearfuel assembly including mating features at a top of the nuclear fuelassembly; a control rod assembly (CRA) inserted in the nuclear fuelassembly with an upper end of the CRA extending out of the top of thenuclear fuel assembly; and a lifting tool including an upper endconfigured for attachment with a crane and a plurality of downwardlyextending elements surrounding an open central region disposed below theupper end, lower ends of the downwardly extending elements beingconfigured to mate with the mating features at the top of the nuclearfuel assembly. The open central region of the lifting tool that issurrounded by the plurality of downwardly extending elements isconfigured to receive the upper end of the CRA when the lower ends ofthe downwardly extending elements mate with the mating features at thetop of the nuclear fuel assembly.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various process operations and arrangements ofprocess operations. The drawings are only for purposes of illustratingpreferred embodiments and are not to be construed as limiting theinvention.

FIGS. 1 and 2 show a nuclear fuel assembly and a nuclear reactor core,respectively, according to the prior art.

FIG. 3 shows a diagrammatic perspective view of a nuclear reactor andselected associated components.

FIG. 4 shows an exploded perspective view of the pressure vessel of thenuclear reactor of FIG. 3.

FIG. 5 shows an exploded perspective view of the lower vessel portion ofthe pressure vessel of FIG. 4 including selected internal components.

FIG. 6 shows a perspective view of a nuclear fuel assembly with the fuelrods omitted to reveal the control rod guide tubes or thimbles, with acontrol rod assembly (CRA) positioned in a withdrawn position above thefuel assembly.

FIG. 7 shows a perspective view of a nuclear fuel assembly with acontrol rod assembly (CRA) inserted in the fuel assembly.

FIG. 8 shows an isolated perspective view of the upper support elementof the CRA of FIGS. 6 and 7.

FIG. 9 shows an enlarged perspective sectional view of the CRA focusingon the upper support element and showing a J-lock coupling between theconnecting rod and the CRA.

FIG. 10 diagrammatically shows a refueling process flow including thoseportions related to unloading spent nuclear fuel assemblies from thereactor.

FIGS. 11, 12, 13, 14, 15, 15A, 16, and 16A show perspective views (withpartial cutaway in the case of FIGS. 15A and 16A) of various operationsof the process flow of FIG. 10.

FIG. 17 shows an illustrative embodiment of the lifting tool includingdiagrammatically indicated motors for rotating the lower ends of thedownwardly extending elements of the lifting tool to engage the locks.

FIGS. 18-20 diagrammatically show overhead views of three nuclear fuelassembly embodiments each with an inserted control rod assembly (CRA)and showing the peripherally located mating features at the top of thefuel assembly for connecting with the lifting tool.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIGS. 3-5, an illustrative nuclear reactor is shown.FIG. 3 shows the nuclear reactor 40 in conjunction with adiagrammatically indicated spent fuel pool 42 and a diagrammaticallyindicated crane 44. FIG. 4 shows an exploded view of the pressure vesselof the nuclear reactor of FIG. 3. The pressure vessel includes a lowervessel portion 50, an upper vessel portion 52, and a skirt or supportstructure 54. In the illustrative arrangement, the pressure vessel ismounted vertically (as shown) with at least part of the lower vesselportion 50 disposed below ground level. The bottom of the skirt orsupport structure 54 is at ground level and supports the pressure vesseland/or biases the pressure vessel against tipping. In the illustrativeexample of FIG. 3, the spent fuel pool 42 is a below-ground poolcontaining water and optional additives such as, by way of illustrativeexample, boric acid (a soluble neutron poison). FIG. 5 shows an explodedperspective view of the lower vessel portion 50 including selectedinternal components. The lower vessel 50 contains the nuclear reactorcore comprising the core former 32 and an array of fuel assemblies 10(only one of which is shown by way of example in FIG. 5). The reactorcore is disposed in and supported by the core former 32 which is in turndisposed in and supported by a core basket 56, which may includeradiation shielding, optional emergency coolant tubing (not shown), orso forth.

The illustrative nuclear reactor includes upper internals 58 whichinclude wholly internal control rod drive mechanism (CRDM) units. In theillustrative example, the upper internals 58 are supported by amid-flange 60 that also forms a structural joint of the pressure vessel(being disposed at the junction between the lower and upper vesselportions 50, 52). Alignment between the fuel assemblies 10 and the upperinternals 58 is suitably provided by the upper alignment pins 26 at thecorners of the upper nozzle plates 22 of the fuel assemblies 10. Thesepins 26 are designed to accommodate the differential thermal expansionbetween the fuel assembly 10 and the reactor internals 58 and the fuelassembly growth due to irradiation without losing engagement.

The illustrative nuclear reactor is a thermal nuclear reactor employinglight water (H₂O) as a primary coolant that also serves as a neutronmoderator that thermalizes neutrons to enhance the nuclear reactionrate. Alternatively, deuterium dioxide (D₂O) is contemplated as thecoolant/moderator. The primary coolant optionally contains selectedadditives, such as optional boric acid which, if added, acts as aneutron poison to slow the reaction rate. The pressure vessel suitablyincludes a cylindrical central riser or other internal compartments orstructures (details not shown) to guide circulation of the primarycoolant in the pressure vessel. The primary coolant circulation may benatural circulation caused by the heating of the primary coolant in thevicinity of the reactor core, or may be assisted or generated byillustrative primary coolant pumps 62 also mounted via the mid-flange60.

Although not illustrated, in some embodiments the nuclear reactor isintended to generate steam. Toward this end, primary coolant heated bythe reactor core flows through a primary loop that is in thermalcommunication with a secondary coolant loop through which secondarycoolant flows. Heat transfer from the primary loop to the secondary loopheats the secondary coolant and converts it to steam. The thermallycoupled primary/secondary coolant loops thus define a steam generator.In some embodiments, the steam generator is external to the pressurevessel, while in other embodiments the steam generator is internal tothe pressure vessel, for example mounted in the upper pressure vesselportion 52 in some contemplated embodiments. The steam may for example,be employed to drive a turbine of a generator of an electrical powerplant, thus generating electrical power from the nuclear reaction.

The illustrative nuclear reactor is of a type generally known as apressurized water nuclear reactor (PWR), in which the primary coolant(water) is maintained in a superheated state during normal operation.This is suitably accomplished by maintaining a steam bubble located atthe top of the upper vessel portion 52 at a desired pressure duringnormal reactor operation. Alternatively, the nuclear reactor could beconfigured as a boiling water reactor (BWR) in which the primary coolantis maintained in a boiling state.

The illustrative nuclear reactor 40 and other components, e.g. spentfuel pool 42 and diagrammatically represented crane 44, is shown as anexample. Numerous variations are contemplated. For example, the pressurevessel can have other portioning, such as having a removable top or“cap” section, and can have access manways provided at various pointsfor maintenance or so forth. In some embodiments the entire pressurevessel may be located underground. Similarly, while the illustrativespent fuel pool 42 is below-ground and surrounds the lower vesselportion 50, more generally the spent fuel pool can be located anywherewithin “reach” of the crane 44, and may in some embodiments beabove-ground (or, conversely, may be buried deep underground withsuitable access from above). The reactor 40 and auxiliary components 42,44 are typically housed in a concrete or steel containment structure,which is also not shown. The crane 44 is diagrammatically shown, and mayin general have any suitable configuration that provides the desiredhorizontal and vertical travel, lifting capacity, and so forth whilefitting within the containment structure. Some suitable craneconfigurations include an overhead crane configuration, a gantry craneconfiguration, a tower or hammerhead crane configuration, or so forth.

With continuing reference to FIGS. 3-5 and with further reference toFIGS. 6-9, reactivity control is suitably achieved using a control rodassembly (CRA) 70 associated with each fuel assembly 10. FIG. 6 shows anillustrative fuel assembly with the fuel rods omitted, denoted byreference number 10′. By omitting the fuel rods for illustrativepurposes, the diagrammatic element 10′ reveals that the control rodguide tubes or thimbles 20 through which neutron-absorbing control rodsmay pass extend through the entire (vertical) height of the fuelassembly. Corresponding control rods 72 of the CRA 70 are shown in thefully withdrawn position in FIG. 6 (that is, fully withdrawn out of theguide tubes or thimbles 20). The CRA 70 also includes upper supportelement 74 that secures the bundle of control rods 72 together in apattern matching that of the guide tubes or thimbles 20. The uppersupport element 74 may be a conventional spider; in the illustrativeexample, however, the upper support element 74 is a larger elementintended to provide various benefits such as a longer (vertical) lengthover which to secure the upper ends of the control rods 72, andoptionally increased mass for the CRA 70. The illustrative upper supportelement 74 is shown in isolation in FIG. 8, and in side sectional viewin FIG. 9. The illustrative upper support element 74 is furtherdescribed in “Terminal Elements for Coupling Connecting Rods and ControlRods in Control Rod Assemblies For a Nuclear Reactor”, U.S. Ser. No.12/862,124 filed Aug. 24, 2010, which is incorporated herein byreference in its entirety. FIG. 7 shows the CRA 70 fully inserted intothe fuel assembly 10. It will be noted in FIG. 7 that a portion of theCRA 70, including at least the upper support element 74, extends abovethe top of the fuel assembly 10 in the fully inserted position.

With continuing reference to FIGS. 6-9, the CRA 70 is inserted into thefuel assembly 10 (as per FIG. 7), or withdrawn from the fuel assembly 10(as per FIG. 6) in order to control the reaction rate of reactivity ofthe reactor core. The control rods 72 comprise a neutron-absorbingmaterial—accordingly, as the control rods 72 are inserted further intothe fuel assembly 10 the reaction rate is reduced. In the fully insertedposition (FIG. 6) the reaction is typically extinguished entirely. Aconnecting rod 76 is employed in order to raise or lower the CRA 70. Asillustrated in FIGS. 6, 7, and 9, the lower end of the connecting rod 76is connected with the upper support element 74 of the CRA 70. Theopposite upper end of the connecting rod 76 is not illustrated, but isconnected with a suitable control rod drive mechanism (CRDM) unit. Inthe illustrative embodiment (see FIG. 5) the CRDMs are wholly internaland are part of the upper internals 58 contained within the pressurevessel. Alternatively, the CRDMs may be mounted externally above thepressure vessel (as is typical in a PWR) or externally below thepressure vessel (as is typical in a BWR), with the connecting rodspassing through suitable vessel penetrations to connect with thecorresponding CRA.

With returning reference to FIGS. 3-5, the reactor core has a sufficientquantity of fissile material to support reactor operation for a designedoperational time period, which is typically of order one to a few years,although shorter or longer designed periods are also contemplated.Thereafter, the nuclear reactor 40 is refueled and then restarted.Toward this end, the crane 44 includes or is operatively connected withlifting tool 80 that is designed to connect with one of the fuelassemblies. During refueling, the crane 44 operating in conjunction withthe lifting tool 80 transfers spent fuel assemblies out of the lowervessel 50 and deposits the spent fuel assemblies in the spent fuel pool42. By way of diagrammatic illustration, FIG. 3 shows several spent fuelassemblies 10 _(spent) which have been transferred into the spent fuelpool 42. (It should be noted that while the illustrative spent fuel pool42 is below-ground and surrounds the lower vessel portion 50, moregenerally the spent fuel pool can be located anywhere within “reach” ofthe crane 44, and may in some embodiments be above-ground.) The crane 44operating in conjunction with the lifting tool 80 also transfers (i.e.,loads) new fuel assemblies into the lower vessel 50, and moreparticularly into the core former 32.

With reference to FIGS. 10-16, the refueling process is described. In anoperation S1, the reactor is shut down preparatory to the refueling. Theshutdown S1 includes inserting each CRA 70 into its corresponding fuelassembly 10, producing the inserted configuration shown in FIG. 7. Asuitable time delay is allowed in order for the reactor to cool down toa sufficiently low temperature to allow opening of the pressure vessel.Some primary coolant may also be removed from the pressure vessel inorder to reduce the water level. In an operation S2 (see also FIGS.3-5), the upper vessel portion 52 is removed (for example, using thecrane 44). The effect of the operation S2 is to provide access to the(now spent) fuel assemblies 10 disposed in the core former 32. In anoperation S3, for each fuel assembly 10 the connecting rod 76 isdetached from the corresponding CRA 70 so as to leave the combination ofthe fuel assembly 10 and the inserted CRA 70, as shown in FIG. 11.

With brief reference to FIG. 9, a suitable approach for performing theremoval S3 of the connecting rod 76 is described. In this embodiment,the lower end 76 _(L) of the connecting rod 76 terminates in a bayonetor (illustrated) J-lock coupling that is designed to lock into a matingreceptacle 76 _(M) (see FIG. 8) of the upper support element 74 of theCRA 70. The perspective sectional view of FIG. 9 shows the lower end 76_(L) of the connecting rod 76 in the locked position biased by a springSS against a retaining feature RR inside the mating receptacle 76 _(M)of the CRA upper support element 74. Thus, by pressing the connectingrod 76 downward against the bias of the spring SS and rotating theconnecting rod 76 to disengage from the retaining feature RR, theconnecting rod 76 is released from the CRA upper support element 74.More generally, a bayonet, J-lock, or other “quick-release” typerotatable coupling can be employed to enable the operation S3 to bequickly performed, with the “groove” and “pin” or other retainingcombination being variously disposed (e.g., with the groove on theconnecting rod and the pin or pins on the CRA receptacle, or viceversa). Some further illustrative description is set forth in “TerminalElements for Coupling Connecting Rods and Control Rods in Control RodAssemblies For a Nuclear Reactor”, U.S. Ser. No. 12/862,124 filed Aug.24, 2010, which is incorporated herein by reference in its entirety.Although a quick-release approach is advantageous, it is alsocontemplated to employ a different approach for performing the operationS3—for example, the connecting rod may be permanently connected with theCRA (for example, by a weld or the like), and the operation S3 mayentail cutting the connecting rod at a point at or near its junctionwith the CRA.

With continuing reference to FIG. 10, after completion of the operationS3 the resulting unit includes the fuel assembly 10 with the CRA 70inserted, with a top portion of the CRA 70 including the upper supportelement 74 extending above the top of the fuel assembly 10. This isillustrated in FIG. 11. In an operation S4 (see also FIG. 12), thelifting tool 80 is lowered onto the top of the fuel assembly 10. As seenin FIG. 12, the lifting tool 80 includes an upper end 81 configured forattachment with the crane 44. In the illustrative lifting tool 80, theupper end 81 includes a loop for attachment with the cable or arm of thecrane 44. The lifting tool 80 also includes a plurality of downwardlyextending elements 82, namely four downwardly extending rods or bars 82in the illustrative example, that surround and vertically overlap theportion of the CRA 70 extending above the top of the fuel assembly 10(e.g., the upper support element 74). The illustrative downwardlyextending elements 82 are vertical rods or bars that are aligned suchthat lower ends 82 _(L) of the downwardly extending elements 82 of thelowered lifting tool 80 align with respective peripherally locatedmating features at a top and periphery of the fuel assembly 10. In theillustrative embodiment, the upper alignment pins 26 of the fuelassembly 10 located at the corners of the upper nozzle plate 22 alsoserve as the mating features 26 (namely lifting pins 26 in theillustrative example) at a top and periphery of the fuel assembly 10.However, other mating features are also contemplated. For example, themating features can be protrusions, openings, or recesses at a top andperiphery of the fuel assembly.

The mating features (e.g., lifting pins 26) are designed to beweight-bearing such that the entire fuel assembly 10 can be raisedupward by lifting on the mating features. In the case of theillustrative fuel assembly 10, this is accomplished by constructing theupper and lower nozzle plates 22, 24, the control rod guide tubes orthimbles 20, and the spacer grid elements 14, 16, 18 as a weldedassembly of steel or another suitable structural material (best seen asthe structure 10′ in FIG. 6). The lifting pins 26 at a top and peripheryof the fuel assembly 10 are secured to the upper nozzle plate 22 bywelding, a threaded connection, a combination thereof, or anothersuitably load-bearing connection.

With continuing reference to FIG. 10 and with further reference to FIGS.14, 15, 15A, 16, and 16A, in an operation S5 the lowered lifting tool 80is connected with the top of the fuel assembly 10. The connectionoperation S5 includes locking the lower ends 82 _(L) of the downwardlyextending elements 80 with the respective peripherally located matingfeatures, e.g. lifting pins 26, at the top and periphery of the fuelassembly 10. In the illustrative approach (see FIGS. 14, 15, 15A, 16,and 16A), the locking operation is performed by rotating at least thelower ends 82 _(L) of the downwardly extending elements 80 to lock thelower ends disposed over (as illustrated) or inside the respectivelifting pins 26 with the respective lifting pins 26. Toward this end,the lower ends 82 _(L) and the respective lifting pins 26 define alockable bayonet coupling. FIG. 14 shows an enlarged view of one of thelower ends 82L aligned with and being lowered over the respectivelifting pin 26. In this view a groove 86 in the lifting pin 26 isvisible, as well as a narrowed portion 88 of the lifting pin 26. Thesefeatures 86, 88 are designed to cooperate with a recess 90 in the lowerend 82 _(L) with a narrowed region 92 to form a rotationally engaginglock. FIG. 15 shows an enlarged view of the lower end 82L fully loweredover the lifting pin 26. FIG. 15A shows the view of FIG. 15 with partialcutaway of the lower end 82 _(L) to reveal internal components of the(unlocked) locking configuration. FIG. 16 shows an enlarged view of thelower end 82L after a rotation of about 90°. This rotation causes thenarrowed region 92 to move into the groove 86 to form the lock. FIG. 16Ashows the view of FIG. 16 with partial cutaway of the lower end 82 _(L)to reveal internal components of the (locked) locking configuration.

In other embodiments, other rotationally locking “quick-release”configurations can be employed. For example, in another contemplatedembodiment the J-lock coupling shown in FIG. 9 for coupling theconnecting rod 76 with the CRA upper support element 74 can be used incoupling the lower end of the downwardly extending rod or bar with amating recess at the top and periphery of the fuel assembly. Anotherrotationally locking quick-release configuration contemplated for use inthe lower ends of the downwardly extending elements of the lifting toolare threaded connections. In this embodiment, the lower ends havethreads that mate with threaded holes located at the top periphery ofthe nuclear fuel assembly. The locking in this case is a frictional lockobtained by rotating the lower ends to thread into the threaded holesuntil a designed torque is reached.

With reference to FIG. 17, in any embodiment employing a rotationallock, the downwardly extending elements, or at least their lower ends,should include motorized rotation capability. In an illustrative exampleshown in FIG. 17, each downwardly extending rod or bar 82 includes adiagrammatically indicated motor 94 providing the motorized rotation ofthe lower end 82 _(L). Although FIG. 17 illustrates a separate motor 94for each downwardly extending rod or bar 82, in other embodiments it iscontemplated to employ a single motor that drives rotation of all lowerends via a suitable drive train (e.g., geared rotating shafts or thelike). It is also noted that since the lifting tool 80 is not disposedinside the pressure vessel except when the reactor is shut down, thelifting tool 80 (including the motors 94) does not need to be rated foroperation at the operating temperature of the nuclear reactor. Themotors 94 should be robust against immersion in the primary coolant andin the fluid of the spent fuel pool 42 (see FIG. 3), for example bybeing hermetically sealed.

While various embodiments of rotational locks (e.g., bayonet or J-lockcouplings) are disclosed herein, other types of locks, includingnon-rotational locks, are also contemplated. For example, in anothercontemplated embodiment the locks may employ motorized clamps that clamponto respective mating features at the top of the fuel assembly.

With returning reference to FIG. 10, in an operation S6 the fuelassembly 10 connected with the lifting tool 80 is moved into the spentfuel pool 42 using the crane 44. In an operation S7 the lifting tool 80is released from the top of the fuel assembly. The release operation S7includes unlocking the lower ends 82 _(L) of the downwardly extendingelements 82 from the respective peripherally located mating features(e.g. lifting pins 26) at the top and periphery of the fuel assembly 10.In the illustrative embodiment, this entails rotating the lower ends 82_(L) in the opposite direction to that used in the locking operation andthen lifting the unlocked lifting tool 80 upward away from the spentfuel assembly now residing in the spent fuel pool 42. Other unlockingoperations may be employed depending upon the nature and configurationof the locking coupling.

Since the reactor core typically includes a number of fuel assemblies 10(see the example of FIG. 2 in which the reactor core 30 includes sixtynine fuel assemblies). Accordingly, after the release operation S7, anoperation S8 is performed in which the next fuel assembly to be unloadedis selected, and the process repeats beginning at operation S4. Once allfuel assemblies have been unloaded, an operation S9 is performed inwhich the lifting tool 80 is parked in a storage location.Alternatively, if new fuel is to be loaded into the reactor, operationsanalogous to operations S4, S5, S6, S7, S8 are performed to pick up newfuel assemblies from a loading dock or other source location and placethe new fuel assemblies into the core former 32, followed by performingcontrol rod reattachment (analogous to operation S3), replacement of theupper vessel portion 52 (analogous to operation S2), and restarting thereactor (analogous to operation S1, and optionally further includingperforming various integrity or safety checks prior to the restart).Note that these analogous loading operations are not shown in FIG. 10.Additionally, the reloading may include performing other maintenancesuch as replacing the connecting rods or other internal reactorcomponents, various inspection and/or cleanup operations, or so forth.

An advantage of the lifting tool 80 is that it accommodates a CRAinserted into the fuel assembly 10 that extends substantially above thetop of the fuel assembly 10. Because no swing action is required toengage the lifting mechanism; the fuel assembly can be lifted even whenmost or all of the inboard volume above the fuel assembly is occupied bythe upper portion 74 of the inserted CRA. The peripherally arrangeddownwardly extending elements 80 accommodate the exposed portion of theCRA by surrounding the exposed upper end of the inserted CRA (e.g., theupper support element 74) when the fuel assembly 10 is connected withthe lifting tool. The downwardly extending elements 82 surround an opencentral region disposed below the upper end 81 of the lifting tool 80,such that the open central region can accommodate the upward extensionof the inserted CRA out of the top of the fuel assembly 10. In this way,the CRA vertically overlaps the lifting tool 80 when the fuel assembly10 is connected with the lifting tool 80 (see FIG. 13). In someembodiments the overlap is at least one-half of the vertical height ofthe lifting tool 80. In some embodiments the overlap between the CRA andthe lifting tool 80 is at least one-half of the vertical height of thedownwardly extending elements 82 of the lifting tool 80.

With reference to FIGS. 18-20, it is to be appreciated that the fuelassemblies, CRA, and lifting tool can have various geometries. FIG. 18shows the illustrative geometry of the fuel assembly 10, which has arectangular cross-section when viewed from above as per FIG. 18, withthe CRA including the upper support element 74 inserted in illustrativeFIG. 18. FIG. 19 illustrates a hexagonal fuel assembly 22 _(H) havingsix sides, with the same CRA including the same upper support element 74inserted. In this embodiment there are six mating features 26 _(H)located at a top and periphery of the fuel assembly. The illustrativesix mating features 26 _(H) are the same as the lifting pins 26 of thefuel assembly 10. The corresponding lifting tool (not shown) suitablyincludes six downwardly extending elements, e.g. six downwardlyextending rods or bars, arranged in a hexagonal pattern to mate with therespective six lifting pins 26 _(H). Finally, as a further example, FIG.20 illustrates a triangular fuel assembly 22 _(T) having three sides,with a conventional spider 74 _(T) with six branches serving as theupper support element of the CRA. In this embodiment there are threemating features 26 _(T), which in this embodiment are embodied asrecesses or openings 26 _(T). The corresponding lifting tool (not shown)suitably includes three downwardly extending elements, e.g. threedownwardly extending rods or bars, arranged in an equilateral triangularpattern to mate with the respective three openings 26 _(T). In general,the geometry of the fuel assembly preferably promotes a closely packedarrangement.

The preferred embodiments have been illustrated and described.Obviously, modifications and alterations will occur to others uponreading and understanding the preceding detailed description. It isintended that the invention be construed as including all suchmodifications and alterations insofar as they come within the scope ofthe appended claims or the equivalents thereof.

1. A method comprising: performing refueling of a nuclear reactor, therefueling including removing a fuel assembly from a reactor core of thenuclear reactor by a removal method including: connecting a lifting toolof a crane with a top of the fuel assembly, the lifting tool comprisingan assembly of downwardly extending elements, the connecting includinglocking lower ends of the downwardly extending elements with respectivemating features located at a top and periphery of the fuel assembly;moving the fuel assembly connected with the lifting tool into a spentfuel pool using the crane; and releasing the lifting tool from the topof the fuel assembly, the releasing including unlocking the lower endsof the downwardly extending elements from the respective peripherallylocated mating features at the top and periphery of the fuel assembly.2. The method of claim 1, wherein the removal method does not includeremoving a control rod assembly (CRA) that is inserted in the fuelassembly and wherein the CRA vertically overlaps the lifting tool whenthe fuel assembly is connected with the lifting tool.
 3. The method ofclaim 2, wherein the overlap between the CRA and the lifting tool is atleast one-half of the vertical height of the lifting tool.
 4. The methodof claim 2, wherein the overlap between the CRA and the lifting tool isat least one-half of the vertical height of the downwardly extendingelements of the lifting tool.
 5. The method of claim 4, wherein thedownwardly extending elements comprise downwardly extending verticalrods or bars.
 6. The method of claim 1, wherein the removal method doesnot include removing a control rod assembly (CRA) that is inserted inthe fuel assembly and wherein the lower ends of the downwardly extendingelements of the lifting tool surround an upper end of the inserted CRAwhen the fuel assembly is connected with the lifting tool.
 7. The methodof claim 1, wherein the peripherally located mating features at the topand periphery of the fuel assembly are selected from a group consistingof protrusions, openings, and recesses.
 8. The method of claim 1,wherein the locking comprises: rotating at least the lower ends of thedownwardly extending elements to lock the lower ends disposed over orinside the respective mating features with the respective matingfeatures.
 9. The method of claim 8, wherein the lower ends of thedownwardly extending elements and the respective mating features definelockable bayonet or J-lock couplings.
 10. A method comprising:performing refueling of a nuclear reactor, the refueling includingremoving a fuel assembly having a control rod assembly (CRA) inserted inthe fuel assembly from a reactor core of the nuclear reactor by aremoval method including: lowering a lifting tool of a crane onto a topof the fuel assembly, the lowered lifting tool including a plurality ofdownwardly extending elements that surround and vertically overlap aportion of the CRA extending above the top of the fuel assembly; lockingthe downwardly extending elements of the lowered lifting tool withcorresponding mating features at the top of the fuel assembly in orderto connect the lifting tool with the fuel assembly; moving the fuelassembly connected with the lifting tool into a spent fuel pool usingthe crane; and disconnecting the lifting tool from the top of the fuelassembly in the spent fuel pool by unlocking the downwardly extendingelements from the corresponding mating features at the top of the fuelassembly.
 11. The method of claim 10, wherein the downwardly extendingelements of the lowered lifting tool vertically overlap the portion ofthe CRA extending above the top of the fuel assembly by at leastone-half of a vertical height of the downwardly extending elements. 12.The method of claim 10, wherein the corresponding mating features at thetop of the fuel assembly are selected from a group consisting ofprotrusions, openings, and recesses.
 13. The method of claim 10, whereinthe locking comprises: rotating at least lower ends of the downwardlyextending elements to lock the downwardly extending elements with thecorresponding mating features at the top of the fuel assembly.
 14. Themethod of claim 10, wherein the locking comprises engaging bayonet orJ-lock couplings between the downwardly extending elements and thecorresponding mating features at the top of the fuel assembly.
 15. Themethod of claim 10, wherein the downwardly extending elements do notpivot during the locking.
 16. The method of claim 10, wherein theplurality of downwardly extending elements consists of at least threedownwardly extending elements.
 17. The method of claim 10, wherein theplurality of downwardly extending elements consists of N downwardlyextending elements where N is at least four.
 18. An apparatuscomprising: a lifting tool including: an upper end configured forattachment with a crane; and a plurality of downwardly extendingelements surrounding an open central region disposed below the upperend, lower ends of the downwardly extending elements being configured tomate with mating features at the top of a nuclear fuel assembly.
 19. Theapparatus of claim 18, further comprising: the nuclear fuel assemblyincluding the mating features at the top of the nuclear fuel assembly.20. The apparatus of claim 19, further comprising: a control rodassembly (CRA) inserted in the nuclear fuel assembly with an upper endof the CRA extending out of the top of the nuclear fuel assembly;wherein the open central region of the lifting tool that is surroundedby the plurality of downwardly extending elements is configured toreceive the upper end of the CRA when the lower ends of the downwardlyextending elements mate with the mating features at the top of thenuclear fuel assembly.
 21. The apparatus of claim 20, wherein thedownwardly extending elements of the lifting tool comprise downwardlyextending rods or bars.
 22. The apparatus of claim 21, wherein thedownwardly extending rods or bars are oriented vertically.
 23. Theapparatus of claim 21, wherein the plurality of downwardly extendingrods or bars consists of at least three downwardly extending rods orbars.
 24. The apparatus of claim 21, wherein the plurality of downwardlyextending rods or bars consists of N downwardly extending rods or barswhere N is at least four.